If the Soviet Union does not drop its demand that British and French missiles be included in negotiations on intermediate-range missiles in Europe, top U.S. officials say they see no way for agreement to be reached in the arms talks under way in Geneva.

Defense Secretary Caspar W. Weinberger, testifying before a Senate committee yesterday, said that if Moscow persists with such demands "it will bring negotiations to a halt."

The Geneva talks began 18 months ago. They are most often described as dealing with roughly 600 missiles the Soviets have in the field targeted mostly on western Europe, and U.S. plans to deploy 572 missiles in western Europe to counter these.

But in fact, officials say the Geneva talks are perhaps fatally hung up on three crucial issues other than that of the missiles:

* The most important and difficult issue revolves around the 162 British and French missiles which the Soviets demand be counted in any assessment of western strength and compensated for by allowing Moscow as many missiles in Europe as the United States, Britain and France combined.

The U.S., French and British governments adamantly refuse to count forces this way. The French and British missiles are sovereign forces, not under NATO control and not for the United States to negotiate away or compensate for, all three governments argue. (In rejecting Andropov's proposal, British Prime Minister Margaret Thatcher yesterday told Parliament that Britain's nuclear force is already "the absolute minimum to deter . . . . ")

The missiles are to defend only Britain and France and are not committed to defend all Europe. But the Soviets say this does not matter, since the missiles are aimed at their territory.

The issue is crucial not so much because of the numbers or military considerations.

Rather, officials say it is because the NATO alliance could crumble if Moscow were to force such concessions. They say they believe the Soviets understand that and Moscow's goal in pressing this point is to split the alliance and provoke a western political disaster which would have longer-term importance than the missiles.

* The second, perhaps equally important issue is the Soviet Union's demand that no agreement on intermediate-range missiles in Europe interfere with its freedom to deploy such weapons in the Asian portions of the country east of the Ural Mountains.

The Soviets now have 351 modern, mobile, triple-warhead SS20 missiles, 243 in Europe and 108 in Asia. More sites in Asia are under construction. Officials say it would be intolerable, in terms of U.S. relations with China and Japan, to allow Moscow to continue building up these weapons in Asia while restricting deployment only in Europe.

Again, the belief is that Moscow, while somewhat concerned about the U.S. missile deployment scheduled to begin in Europe in December if no agreement is reached before then, is really more concerned with achieving results which could split the United States and its allies.

The United States is insisting that all SS20s, no matter where they are based, be included in a ban or reductions. They say the SS20 may have a longer range than the 2,700 to 3,000 miles now officially estimated so that it might reach Europe from Asia.

They also say a significant number of SS20s could be moved from east to west in a few weeks in a crisis and probably all of them moved from Asia within a month.

Whether NATO could react in time to counter that, or could suddenly bring in new missiles, is improbable, they said.

* The third key issue involves not missiles but aircraft capable of carrying atomic weapons. Moscow is demanding big reductions in such U.S. air forces. But Americans argue that virtually all these planes, while capable of carrying atomic bombs, are primarily for fighting conventional wars.

Thus, a Soviet success on this score would also be disastrous for the alliance, they say.

It is against this gloomy background that Paul H. Nitze, chief U.S. negotiator at the intermediate-nuclear forces (INF) talks, will return to Geneva for the next round that opens May 17.

On March 30, President Reagan altered his original negotiating position on intermediate-range missiles. He had urged that both sides eliminate all such weapons.

He said he would be willing instead, on an interim basis, to have both sides reduce deployed or planned forces to equal levels.

It is expected to take American negotiators about a month to lay out this new U.S. plan in detail and explore initial Soviet reactions.

The initial reaction of Soviet Foreign Minister Andrei Gromyko on April 1 plus later statements by Soviet leader Yuri V. Andropov, however, have not been encouraging, officials say. It is not expected that the Soviet negotiator will have anything new to say initially beyond what his leaders have already said in public, although it is possible that after the U.S. ideas are laid out there could be new instructions from Moscow.

On Tuesday, Andropov said he was willing to negotiate equal numbers of warheads as well as missiles. But he also demanded French and British warheads be counted and essentially excluded any new U.S. missiles. Reagan said he was encouraged by the willingness to use warheads as a measurement. But U.S. officials stress that there was no indication of movement on the three main problems.

Officials here also suspect there is nothing new in a Soviet press agency followup yesterday to Andropov's offer. It suggested arrangements could be made to control movement of missiles from Europe to Asia. Specialists here say they want missiles dismantled rather than moved and that the uncertain range of the SS20 means it might still reach Europe.

Meanwhile, the administration here has come under intensive pressure to reach arms control agreements. This comes in the form of a new pastoral letter from Catholic bishops, letters from influential lawmakers to the president and a House vote for a qualified nuclear freeze. The freeze resolution, however, contains an amendment that would allow the planned U.S. missile deployment in Europe if no agreement is reached in Geneva by December.

If there is no agreement and the United States begins to field its new missiles, specialists here say they do not believe Moscow will break off the talks, although they don't exclude such action. Given the political controversy in western Europe over the forthcoming deployment of the new U.S. weapons, the Soviets may figure the missiles will be less dangerous than valuable as sources of continuing divisiveness in the West after they are installed.

There also seems little immediate prospect that the INF talks could be merged into the other negotiation in Geneva known as START on the longer-range strategic forces of the two superpowers. Some officials, however, see this as a future possibility which would cover greater numbers of missiles and thus provide more room for maneuver on both sides.

Talk of this now, however, would weaken western resolve to go ahead with the immediate deployments scheduled for December, some specialists say.